Mammal eggs and reptile placentas

Summary of problems:

There are mammals that lay eggs. There are reptiles that have a rudimentary placenta. A May, 2007 errata from the authors corrects only one of the four major errors in these two sentences.

Full discussion:

This is one of the most baffling claims in the entire textbook. Here is the original text from EE:

Reptiles and mammals reproduce very differently. Most reptiles lay eggs, while mammals carry fertilized eggs internally in a placenta and bear live young.

EE, p. 129

An errata sheet, dated May, 2007, came with the first releases of EE. It made a correction to this statement (correction in bold).

Reptiles and mammals reproduce very differently. Most reptiles lay eggs, while mammals carry fertilized eggs internally, which they nourish through a placenta, and bear live young.

EE, errata sheet of May 2007

These two sentences in EE, and the botched correction, provide perhaps the clearest evidence of the distance between the authors of this book and mainstream science, basic biological knowledge, and science pedagogy.

This correction addressed only one of the many things wrong with this statement. Indeed, the uterus (not the placenta) is the organ in which mammals carry their fertilized eggs; the placenta is the source of nourishment for these internally carried young in most mammals. It is gratifying to note that this basic biological fact is now correct in the textbook, but it is actually quite puzzling that such an error could have escaped the attention of even the most cursory reviewer.

Three other errors, however, remain uncorrected. The sentences (1) ignore other reproductive modes of mammals, (2) downplay the rich diversity in reptilian reproduction, and (3) imply the nonexistence of intermediate reproductive modalities.

The marsupial mammals have a rudimentary and short-lived placenta which is, in most marsupials, structurally and functionally different from the typical eutherian placenta. Placental nourishment of marsupial young is negligible compared to nourishment from the milk obtained in the pouch. Furthermore, there are mammals which lay eggs and have no placenta. These creatures, the monotremes, share with other mammals the characteristics of fur and the ability to lactate, but they lay eggs with leathery shells, which the females then incubate in a pouch.

Some reptiles (e.g. garter snakes) are viviparous and develop a rudimentary placenta (see Stewart, JR, American Zoologist 1992 32(2):303-312, "Placental Structure and Nutritional Provision to Embryos in Predominantly Lecithotrophic Viviparous Reptiles" for a not-so-recent discussion of these facts).

It is also quite telling that these variations on reproductive physiology in both reptiles and mammals are more evidence that there are identifiable transitions in form and function among living organisms. In fact the placenta, in all of its variants, is functionally and structurally derived from membranes found in eggs; the transition between reptiles and mammals becomes obvious when all of the data are considered. A "lack of transitional forms", as noted elsewhere in this review, is a standard creationist canard, but in this book transitional forms are lacking only because the authors choose to ignore them.

In fact, the living mammals illustrate a clear transition from egg-laying through various transitional stages along the way to live birth as seen in humans. In monotremes, which consist of the platypus and spiny anteaters called echidnas, the reproductive system is, as James Vaughan explains in his textbook Mammalogy, "a mix, including primitive features shared with amniotes and unique specializations." Monotreme eggs are structurally more similar to reptile and bird eggs than to eggs of other mammals, and have texture similar to reptile eggs as well. Monotremes, like reptiles, have a structure called a cloaca, in which the reproductive, urinary and digestive tracts all exit through a single opening, rather than through two, a trait shared with birds, reptiles and marsupials, but not with the rest of the mammals. It is a small step from laying these sorts of eggs to the marsupial system of briefly holding the developing embryo internally and nursing the partially developed embryo externally, then successively modifying the placental interface between fetus and mother until we see the sort of live birth found in humans, other eutherian mammals, and even a few transitional marsupials.

This sequence illustrates several important errors in Explore Evolution. Clearly, EE badly misrepresents mammalian reproduction. More fundamental, and more widespread, is its tendency to treat a large taxonomic group of species as if they are all practically identical. Species do differ one from another, and in important ways. Those differences are essential to evolutionary processes, and understanding such variation is vital to a student's understanding of evolution. By teaching students that it is acceptable to treat mammals or reptiles or other groups as if all members of the group were interchangeable does students a disservice and misinforms them about the range of adaptations which exist in the natural world, and how the variation among living things reveals evolutionary processes. Only by obscuring legitimate science can Explore Evolution create the false impression that there are unbridgeable gaps between major taxonomic groups; gaps which bolster their preconceptions against common descent.

These three errors — and the original eggs-in-placenta error — could have been avoided if the authors consulted a standard college-level introductory textbook, or if they were familiar with basic biological literature, or even if they had used used basic resources like Wikipedia, Google, or even a smart 5th grader.